1. Field of the Invention
The present invention relates to an electrically heated window, and more particularly but not exclusively to a laminated vehicle window comprising a number of plies, the window including an array of fine, closely spaced wires extending over one of the plies and forming a heating element, the heating effect resulting from passage of electric current through the wires.
2. Description of the Related Art
Such windows may be installed as the windscreen, rear window, or other window of a car or other vehicle, or as a window (especially the windscreen) of a commercial vehicle, locomotive or aircraft, or in a boat or ship. The electric heating is used to disperse condensation or ice on the surfaces of the window, and thereby maintain good visibility through the window.
At its simplest, a laminated window comprises an inner ply of interlayer material ("the interlayer") disposed between two outer plies of rigid transparent sheet glazing material, but more sophisticated laminated windows comprise greater numbers of plies of interlayer and glazing material, for example so as to confer greater resistance to impact damage upon the window, and reduce the risk of penetration of the window. The interlayer is normally a flexible plastics material, e.g. polyvinylbutyral, and the glazing material may be glass or a rigid plastics material. Laminated windows are also known comprising two or more plies, in which an exposed outer ply is a ply of flexible plastics material which may both increase penetration resistance and reduce the risk of laceration by fragments of glazing material during an impact. Such a plastics outer ply normally forms the inward-facing surface of the window, and is generally subjected to a surface treatment to increase its abrasion resistance.
An electrically heated window comprising an array of fine, closely spaced wires is known from U.S. Pat. No. 3,729,616. This document describes a laminated window having a pattern of wrinkled resistance wires with the wrinkles oriented in non-parallel random fashion so as to reduce glare from the wires when the window is used as a windscreen. The heated area is generally rectangular, and since a windscreen is normally approximately the shape of a trapezium, there are generally triangular areas adjacent each short edge of the windscreen which are unheated. Obviously, any condensation or ice in these unheated areas will remain in place, impairing visibility through the windscreen. In the present specification, "trapezium" is used to mean a quadrilateral with only one pair of sides parallel.
In modern windscreens, the unheated triangular areas are substantial, and may even include part of the area normally wiped by the windscreen wipers (the "wiped area"). The resulting impairment of visibility is most undesirable, and may have serious safety consequences. Furthermore, when ice remains present in the wiped area, damage to the wiper blades may occur as a result of scraping over the ice. A prior art windscreen, showing such unheated areas, is illustrated in FIG. 1 of the present application; it would be desirable to be able to heat such areas also.
One possible solution is described in GB 1,365,785, which is primarily concerned with the problems of electrical discontinuities and resistance varying with path length, but also discloses an arrangement which heats substantially the whole of two particular shapes of window. Two windows, in the shape of a rectangle and a trapezium respectively, are described in which the wires run from side to side, i.e. parallel to the two long, parallel, edges. If the window were a windscreen, the wires would therefore be parallel to the top and bottom edges. The wires are arranged in a number of "elements", each with its own "collectors", and these elements are connected in series so that the varying resistances of the elements are compensated for, and the same current flows through each element. The complexity of the circuit would however make it expensive to manufacture.
Moreover, this arrangement does not solve the problem of extending the heated area in most modern vehicle windows, which have curved edges and so are only approximately the shape of a trapezium. GB 1,365,785 only shows how to extend the heated area in windows of the particular shapes and with wires of the particular orientations disclosed, i.e. parallel to the long edges of a window. In actual fact, when the window is a windscreen, it is greatly preferred to arrange the wires to run at right angles to the two long, generally parallel, edges, i.e. from top to bottom. This arrangement facilitates the heating of all of the primary vision area (as defined in safety standards for vehicles), and reduces the optical interference to vision caused by the wires, especially as windscreens are installed at increasing rake angles, i.e. closer to the horizontal. A more general approach to the problem of extending the heated area of vehicle windows is therefore needed.
EP 32,139, which corresponds to U.S. Pat. No. 4,395,622, describes a solution to the subsidiary problem of part of the wiped area being unheated. The wires are laid along circular arcs extending in the direction of movement of the wiper. The patent teaches that the heated area should be arranged to approximately coincide with the wiped area; a substantial portion of the window is therefore left unheated as in U.S. Pat. No. 3,729,616. The patent does not therefore address the problem of extending the heated area beyond the wiped area.
GB 1,566,681, corresponding to U.S. Pat. No. 4,209,687, addresses the problem of diffractive effects occurring as a result of light passing through fine, closely spaced wires in an electrically heated window. It is suggested that each wire is formed as a helix to alleviate such effects. Neighbouring helices may be disposed in parallel, trapezoidal, meandering or undulating relation according to the pattern desired. However, no indication is given as to how the helices are formed, how they are laid down in these various relationships, or why a particular pattern may be desired, and so this document does not assist the skilled person seeking to extend the heated area of a window.